1. Field of the Invention
This invention generally relates to a light emitting device, and, in particular, to a semiconductor light emitting device, such as a light emitting diode or semiconductor laser, for emitting light in a direction perpendicular to a main surface of a substrate. More specifically, the present invention relates to a semiconductor laser device capable of emitting a laser beam in a vertical direction with respect to a main surface of a substrate.
2. Description of the Prior Art
A semiconductor light emitting device, such as a light emitting diode or semiconductor laser, is well known in the art, and it generally includes a semiconductor substrate in which one or more PN junctions are provided to define an activation region for emitting light. A semiconductor light emitting device capable of emitting light in a vertical direction with respect to a main surface of a substrate is also known and this type of semiconductor light emitting diode is preferred because it can be easily coupled to a light transmitting element, such as an optical fiber, and it has various possible applications, such as a display device.
FIGS. 1 and 2 show typical prior art semiconductor light emitting devices capable of emitting light in a vertical direction. A light emitting device shown in FIG. 1 includes a semiconductor substrate 2 having an electrode 1 attached to its bottom. The substrate 2 includes a p-AlGaAs layer 21, a p-AlGaAs activation layer 3 and an n-AlGaAs layer 22 from bottom to top, and another electrode 5 circular in shape is provided on the layer 22. On the other hand, FIG. 2 shows a prior art light emitting device which includes a semiconductor substrate 2 having a bottom electrode attached to the bottom thereof. The substrate 2 includes an n-GaAs layer 24, an n-GaAsP graded layer 25 and an n-GaAsP layer 26 from bottom to top, and a circular top electrode 5 having a light emitting opening 51 is provided on layer 26. And, an activation layer 3 is formed inside of the n-GaAsP layer 26 at a location immediately below the light emitting opening 51.
As is obvious from FIGS. 1 and 2, even if it is of the vertical light emitting type, any prior art semiconductor light emitting device includes one or more PN junctions defined in a substrate to define an activation region, where the light-emitting phenomenon takes place, as extending in parallel with a main surface thereof, which is a surface having a larger area. And, the activation region typically has a thickness in the order of 2-3 microns. As a result, in order to attain a sufficiently large gain in the intensity of light emitted in a direction vertical to the main surface of the substrate, an extremely large injection current density is required. Since the activation layer 3 extends laterally or in parallel with the main surface and the light emitting function takes place across the entire activation layer 3, the light emitted in the direction perpendicular to the main surface of the substrate is lower and limited in light intensity.